The use of isotope tracers to study metabolism has long been known. Samples of body fluids or exhaled gas are typically collected from the subject at discrete intervals at times after administration of isotopically labeled (e.g., 2H, 13C, 15N, 18O) substrate for metabolism by the subject (e.g., amino acids, sugars, fats). Exhaled gases can be collected using an open circuit device in which the subject inhales air through a mouthpiece from a defined source, and the exhaled air is collected. A similar open circuit method can be used in an oxygen tent like setting in which air is delivered in a controlled manner to the chamber, but because the circuit is open, volumes and concentrations of inhaled and exhaled gases are harder to precisely measure. The amount of isotope present in the sample can be determined using known methods such as Isotope Ratio Mass Spectroscopy (IRMS). The sample is processed before entering the mass spectrometer so that only a single chemical species enters at a given time. Generally, samples are combusted or pyrolyzed and the desired species (usually hydrogen gas H2, nitrogen (N2), carbon dioxide (CO2), or sulfur dioxide) is purified by means of traps, filters, catalysts and/or chromatography.
The two most common types of IRMS instruments are continuous flow and dual inlet. In dual inlet IRMS, purified gas obtained from a sample is alternated rapidly with a standard gas (of known isotopic composition) by means of a system of valves, so that a number of comparison measurements are made of both gases. In continuous flow IRMS, sample preparation occurs immediately before introduction to the IRMS, and the purified gas produced from the sample is measured just once. The standard gas may be measured before and after the sample or after a series of sample measurements. While continuous-flow IRMS instruments can achieve higher sample throughput and are more convenient to use than dual inlet instruments, the yielded data is of approximately 10-fold lower precision. Such devices and methods are well known to those of skill in the art (see, e.g., U.S. Pat. No. 6,486,474, incorporated herein by reference).